PROJECT SUMMARY MPS1 encodes a conserved kinase that is essential for multiple cell cycle related functions. One critical role for Mps1 is to promote the formation of force-generating associations of kinetochores with microtubules in meiosis. These connections are critical for chromosomes to become properly oriented on the meiotic spindle, but the molecular mechanisms by which Mps1 forms these associations is a mystery. Errors in meiotic chromosome bi-orientation lead to gametes with the wrong number of chromosomes, referred to as aneuploidy, which in humans is the leading cause of birth defects and mental retardation. Mps1 is necessary to prevent this aneuploidy, as loss of Mps1 leads to high levels of aneuploid gametes. Mps1 is also implicated in human cancer. Many human cancers are aneuploid, with very high chromosome numbers, and over-express Mps1. Aneuploid breast tumor cells are especially dependent upon Mps1 ? presumably because of the demands placed upon the bi-orientation machinery by the high chromosome numbers in these cells. These results have led to the pursuit of anti-Mps1 compounds that might act as anti-tumor drugs. Determining the mechanism-of-action of Mps1 in bi-orientation would clarify the specific Mps1 protein interactions that might be the best targets for the development of refined anti-tumor therapeutics. This project seeks determine the mechanisms used to form force-generating kinetochore-microtubule attachments in yeast meiosis, and the role of Mps1 in that process. The project has four Aims. The first will use imaging approaches to monitor the interactions between a centromere and a single microtubule, to pinpoint the step in developing kinetochore-microtubule attachments that is defective in mps1 mutants. A second Aim will explore the roles of known phosphorylation targets of Mps1 that reside at the kinetochore-microtubule interface, in the formation of productive kinetochore-microtubule attachments. This aim will be supported by a collaboration with investigators who will uncover new phosphorylation targets of Mps1 by mapping the Mps1 phospho-proteome. A third Aim is to characterize a collection of strains that carry suppressor mutations that improve chromosome segregation in mps1-R170S mutants ? these suppressor mutations likely map to genes that are involved in kinetochore-microtubule interactions and their analysis will shed light on how Mps1 is regulating this process. The final objective is to determine whether Mps1 contributes to mitotic chromosome segregation in the same way that it promotes proper chromosome segregation in meiosis. Because the proteins being examined in this proposal are for the most part shared between yeast and humans, the insights gained in the yeast system could have important implications for better understanding the origins aneuploidy and treatment of cancer in humans.